1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate whereby a negative-working photopolymer printing plate precursor is image-wise exposed, treated with a gum solution in a gumming station including a first and at least a second gumming unit, whereby, in the first gumming unit, at least a portion of the top layer is removed and whereby, in the second gumming unit, the non-exposed areas are subsequently removed and the plate is gummed in a single step.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout, and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called a plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided into three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light; and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Photopolymers can be sensitized for blue, green, or red light (i.e., wavelength range between 450 and 750 nm), for violet light (i.e., wavelength range between 350 and 450 nm) or for infrared light (i.e., wavelength range between 750 and 1500 nm). Laser sources have been increasingly used to expose a printing plate precursor which is sensitized to a corresponding laser wavelength. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been achieved using an InGaN material. An infrared laser diode emitting around 830 nm or a Nd-YAG laser emitting around 1060 nm can also be used.
Typically, a photopolymer plate precursor includes a support, a photopolymerizable coating, and an overcoat. The photopolymerizable coating includes a polymerizable compound, a polymerization initiator, and a binder and the overcoat usually includes a polyvinylalcohol binder to hinder the penetration of oxygen in the coating. Upon image-wise exposure, free radicals formed by the initiator are not quenched by the oxygen and can initiate crosslinking and/or polymerization of the polymerizable compound, resulting in hardening or curing of the exposed areas. The exposed precursor is usually processed in an alkaline developer having a pH>10, whereby, besides the photopolymerizable coating at the non-exposed areas, the overcoat of the precursor is also solubilized in the developer solution. U.S. 2004/0131974 discloses a method for making such a lithographic printing plate whereby, after image-wise exposure, the overcoat is removed with water in a pre-washing step, resulting in a reduction of the formation of sludge in the alkaline developing solution and whereby staining is prevented at the non-image portions.
Currently, most commercial lithographic plates require an additional gumming process after the exposed plate is developed and before it is put on the press, in order to protect the plate from contamination, e.g., by oxidation, fingerprints, fats, oil or dust, or from damaging, e.g., by scratches during handling of the plate. Such an additional gumming step is not convenient for the end-user because it is a time consuming step and requires an additional gumming station.
WO 02/101469 discloses a method of processing an imageable element useful as an alkaline-developable lithographic printing plate precursor wherein the element is developed and gummed with an aqueous alkaline developing-gumming solution including a water-soluble polyhydroxy compound having a specific structure.
EP 1 342 568 discloses a method for making a heat-sensitive lithographic printing plate wherein the image-wise heated precursor, including a coating of hydrophobic thermoplastic polymer particles which coalescence on heating, is developed with a gum solution. A practical preferred embodiment for this type of printing plate was introduced by Agfa under the trade name Azura.
In U.S. Pat. Nos. 6,027,857, 6,171,735, 6,420,089, 6,071,675, 6,245,481, 6,387,595, 6,482,571, 6,576,401, and 6,548,222, a method is disclosed for preparing a lithographic printing plate wherein a photopolymer plate, after image-wise exposure, is mounted on a press and processed on-press by applying ink and fountain solution to remove the unexposed areas from the support. Also, U.S. 2003/0016577 and U.S. 2004/0013968 disclose a method wherein a plate including a photopolymerizable layer can be processed on-press with fountain solution and ink or with a non-alkaline aqueous developer. An adhesion promoting compound can also be added to the printing plate precursor for improving the developability of the on-press processing and for improving the durability of the plate in the printing process. Typically, these compounds have an ethylenically unsaturated bond and a functional group capable of adsorbing to the surface of the support. Other compounds and polymers can be used as an adhesion promoting compound. The compound can be present in the photopolymerizable layer or in an intermediate layer between the support and the photopolymerizable layer as disclosed in EP 851 299, EP 1 091 251, U.S. 2004/0214105, EP 1 491 356, U.S. 2005/0039620, EP 1 495 866, EP 1 500 498, EP 1 520 694, and EP 1 557 262.
A first problem associated with on-press processing of such photopolymer printing plates is the lack of daylight stability, i.e., the image is not stable before processing and, therefore, the exposed plate needs to be processed within a short time after the exposure. However, since on-press processing is not possible during a print job, the end-user must wait until the previous print job has been completed before the exposed plate can be mounted on the press and processed. As a result, the exposure of the plate for the next print job must be delayed until just before the completion of the previous print job, so as to avoid that the unprocessed plate is affected by the ambient light. Alternatively, the exposed plate must be kept under safe-light conditions, but this again reduces the ease of use and convenience that are normally associated with, e.g., violet- and infrared-sensitive photopolymer plates.
A second problem left unsolved in the prior art about on-press processable photopolymer plates is the lack of a visible image between exposure and processing. Although it is known to add a colorant to the photosensitive coating, so as to obtain a visible image after removal of the non-exposed areas of the coating by the processing, this does not make it possible to distinguish an exposed plate from an unexposed plate immediately after the image-wise exposure, let alone to inspect the image quality after the exposure, because the visible image is only revealed after the on-press processing. Moreover, on-press processable plates normally do not contain a colorant because the on-press removal of the non-printing areas of the coating may cause contamination of the fountain solution and/or the ink and it may take an unacceptable number of printed copies before the contamination by the colorant has disappeared.
A third problem associated with on-press processing with fountain solution and ink is an insufficient clean-out of the non-exposed areas.
In WO 2005/111727, a method for making a lithographic printing plate is disclosed wherein the image-wise exposed precursor is developed with a gumming solution.